A foldable bicycle is different from a non-foldable bicycle in that the former has a frame that can be folded to reduce its length and volume so as to facilitate its storage and transportion. The prior art foldable bicycle frame is provided at an appropriate location thereof with a joint permitting the frame to be folded in such a manner that the front wheel and the rear wheel are arranged side by side or near to each other. It is expensive to build a foldable bicycle in view of the fact that the foldable bicycle has a folding joint of complex construction and vulnerable to becoming loosened. For this reason, the frame of the foldable bicycle is relatively less able to withstand a percussive force. With a view to overcoming such a structural deficiency of the prior art foldable bicycle frame, this applicant has disclosed a number of improved foldable bicycle frames, as exemplified in a U.S. Pat. No. 5,222,751. However, such improved frames disclosed by this applicant are by no means free from the structural deficiency.